1. Field of the Invention
The present invention relates to a swirler for forming a premixture in a pre-mixed flame type low NOx combustor of a gas turbine.
2. Description of the Prior Art
The above-mentioned type of combustor, which is known in the prior art, will be outlined below with reference to FIGS. 2 to 5. FIG. 4 is an entire constructional view of one example of a prior art pre-mixed flame type low NOx combustor and FIG. 5 is a cross sectional view of a main fuel nozzle as part of the combustor of FIG. 4.
What is called a fuel supply nozzle for supplying fuel and air or a mixture thereof into a combustor consists of a pilot fuel nozzle 204 for forming a flame portion in a cross sectional center of the combustor, and a plurality of main fuel nozzles 202. Each of the main fuel nozzles has an outer casing 206. The main fuel nozzles are arranged so as to surround the pilot fuel nozzle 204 for forming a pre-mixed gas of fuel and air, etc.
In an upstream portion of each of the main fuel nozzles 202, there is provided a main swirler 201 surrounding the main fuel nozzle 202 and extending to a position adjacent the outer casing 206.
Also, in a wall of the body of the main fuel nozzle 202 on a downstream side of the main swirler 201, there are bored a plurality of nozzle holes 205 along a circumferential direction of the main fuel nozzle 202.
In the prior art combustor constructed as above, the main swirlers 201 are provided in plural units and a pilot swirler 203 in a single unit at a center of the combustor, and combustion air is supplied through the plurality of main swirlers 201 and the pilot swirler 203 and fuel is supplied from the plurality of main fuel nozzles 202 and the pilot fuel nozzle 204.
In the main fuel nozzle 202, as shown in FIG. 5, the fuel is injected from the nozzle holes 205 bored in the wall of body of the main fuel nozzle 202 and is mixed with the air flowing on an outer periphery of the nozzle via the main swirler 201 to form a pre-mixed gas.
When the air flows through the main swirler 201, it is given a swirling angle by the main swirler 201 and this angle is governed by a fitting angle in which a swirler vane is fitted to a hub portion thereof relative to a center axis of the fuel nozzle.
In the prior art swirler, while there is seen such an example that the fitting angle of the swirler vane is changed and adjusted for changing the swirling angle, the swirler in actual use remains such that when the fitting angle of the swirler vane to the hub portion (hub portion fitting angle) is changed and adjusted, that hub portion fitting angle is maintained the same as far as to a tip portion of the swirler vane and there is seen no more example of angle change.
FIGS. 2 and 3 show concrete examples of changing the hub portion fitting angle of the swirler vane.
That is, as shown in FIGS. 2(a) and 2(b) one example is that the hub portion fitting angle of a swirler vane 201a relative to a center axis C of the main fuel nozzle 202 is 25xc2x0. In particular, FIG. 2(a) is a view showing an arrangement of a swirler relative to the fuel nozzle and FIG. 2(b) is a view showing an arrangement of the swirler vanes.
Also, another example is that the hub portion fitting angle of a swirler vane 201b relative to the center axis C of the main fuel nozzle 202 is 45xc2x0, as shown in FIGS. 3(a)-3(b). In particular, FIG. 3(a) is a view showing an arrangement of a swirler relative to the fuel nozzle, and FIG. 3(b) is a view showing an arrangement of the swirler vanes.
In either of FIGS. 2 and 3, air A supplied from upstream runs into the swirler vanes 201a or 201b to form an outward swirling flow and fuel F of natural gas and the like is supplied into this swirling flow of air via nozzle holes 205 of the main fuel nozzle 202 to form a pre-mixture of the fuel F and the air A.
In the prior art swirler in which the hub portion fitting angle of the swirler vane 201b shown in FIG. 3 is 45xc2x0, because the angle is as large as 45xc2x0, the shearing flow of the air A is strong, so that mixing of the fuel F and the air A is accelerated very favorably.
However, due to the strong shearing flow, there is formed a large stagnation point P at a tip portion of the main fuel nozzle 202, as shown by a hatched portion in FIG. 3(a), and if a back fire phenomenon once occurs, flame stagnates at the stagnation point P, so that there arises a problem that the main fuel nozzle 202 is apt to burn.
On the other hand, in the prior art swirler in which the hub portion fitting angle of the swirler vane 201a shown in FIG. 2 is 25xc2x0, because the angle of 25xc2x0 is comparatively small, shearing flow of the air A is not so strong and the stagnation point P which is formed at the tip portion of the main fuel nozzle 202, as shown by a hatched portion in FIG. 2(a), is small, hence even if a back fire phenomenon occurs, flame does not specifically stagnate at the stagnation point.
However, this effect is obtained by the shearing flow of the air A which is not very strong and as a result, mixing of the fuel F and the air A, which is a function required for a pre-mixed type combustor, becomes worse, as clearly understood when compared with the swirler of FIG. 3 in which the hub portion fitting angle of the swirler vane 201b is 45xc2x0 and there is a problem of narrow range of condition within which a low NOx combustion is attained.
It is therefore an object of the present invention to provide a combustor with a swirler which is able to accelerate mixing of fuel and air as well as to reduce a stagnation point formed at a tip portion of a main fuel nozzle to thereby prevent the occurrence of flame stagnation and burning of components so as to solve the problems in the prior art.
In order to attain this object, the present invention provides a three-dimensional swirler characterized in that a swirler vane fitted around a fuel nozzle is twisted from a hub side thereof to a tip side thereof so that a fitting angle of the tip side relative to a center axis of the fuel nozzle is larger than that of the hub side.
The present invention is directed to the pre-mixture-forming swirler in a pre-mixed flame type combustor of a gas turbine. The swirler vanes can accelerate the mixing of fuel and air as well as stabilize the flames and prevent the occurrence of burning damage of the nozzles. The swirler is constructed in a three-dimensional structure having the swirler vanes twisted from the hub side to the tip side, wherein the tip side has the fitting angle that is larger than the hub side with respect to the central axis of the fuel nozzle. That is, the fitting angle on the hub side is 25xc2x0 or less so that the flame stagnation point formed in the tip portion of the main nozzle may be made smaller and the fitting angle on the tip side is 25xc2x0 or more so that fuel and air may be mixed together with the shear flow thereof being strengthened. Further, the preferred angle on the hub side is 25xc2x0 and the preferred angle on the tip side is 45xc2x0.
Furthermore, the combustor comprises the pilot fuel nozzle for forming a flame portion in the central portion of the combustor and the main fuel nozzles, each nozzle including a cylindrical outer casing, are arranged so as to surround the pilot fuel nozzle for forming a pre-mixed gas of fuel and air. The swirler is arranged so as to surround each of the main fuel nozzles and extend to the position of the outer casing. Each of the main fuel nozzles further comprises the nozzles holes bored therein along the circumferential direction of the main fuel nozzle downstream of the swirler. In such a combustor, in the prior art case, the swirler vane angle has been the same at both the tip side and the hub side, so that if the mixing of fuel and air is to be improved, that is, if the vane angle is made larger, a stagnation portion is formed at the tip end portion of the main fuel nozzle and, thereby if a back fire phenomenon occurs, the flame stagnates at the stagnation portion and there arises a problem in that the main fuel nozzle is apt to burn. On the other hand, if the vane angle is made smaller for the purpose of solving the burning problem of the main fuel nozzle, then the mixing of fuel and air becomes insufficient. The present invention solves these mutually contradictory problems at one time. In the present invention, the swirler is made in the three-dimensional structure such that the vane angles at the hub side and the tip side are different, that is, the angle on the hub side is 25xc2x0 or less in view of the burning problem and the angle on the tip side is 25xc2x0 or more in view of the mixing acceleration.